The present invention relates to a process and to an apparatus for automatic, continous, destruction-free checking of stoneware pipes and similar objects with resonance frequency vibrations.
There has already been made an endeavor to test stoneware pipes without destruction using resonance frequency vibration measurements with low-frequency mechanical waves. The pipe was positioned over a vibrator, where the frequency was generated by a sound generator internally located in the center or middle of the pipe and was transmitted laterally to the wall of the stoneware pipe. The pipe stood vertically with its spigot end on a foam rubber mat. The spigot end, as defined in Webster's unabridged New International Dictionary, is the male end of a section of pipe which enters the hub end of the next section. The frequency was picked up with a sound pickup device positioned on the oppositely situated internal wall of the pipe, likewise being within the middle of the pipe. The measured resonance frequencies were, among other things, brought into correlation with the crushing strength of a certain type of pipe. As is well known in the art, the correlation of the crushing strengths to the resonance frequency vibrations is dependent upon or influenced by many variable factors such as the production material, the degree of firing, the nominal width, the length, the body thickness, as well as the porosity and moisture content of the pipes, so that assured interrelations or calibrations thereof cannot be guaranteed.
The known process, however, was unable to establish itself for quality monitoring in the stoneware industry, especially because of the position of the pipe, the type of vibration stimulation, the positioning of the vibration measuring heads within the pipe, as well as the sensitivity of the apparatuses to any interference sound. Thus, a dependable allocation of the measured resonance frequency vibrations to a particular pipe type was not possible. Moreover, the known process is not suited for automation which provides a continuous quality monitoring because the handling and centering of the measuring instruments for coupling to the vertically standing pipes, especially in the testing of pipes of differing nominal diameters, require considerable application of machinery.